FFI (C89) Interface

New in version 1.11.14.

Botan’s ffi module provides a C89 API intended to be easily usable with other language’s foreign function interface (FFI) libraries. For instance the included Python wrapper uses Python’s ctypes module and the C89 API. This API is of course also useful for programs written directly in C.

Code examples can be found in src/tests/test_ffi.cpp.

Versioning

uint32_t botan_ffi_api_version()

Returns the version of the currently supported FFI API. This is expressed in the form YYYYMMDD of the release date of this version of the API.

int botan_ffi_supports_api(uint32_t version)

Returns 0 iff the FFI version specified is supported by this library. Otherwise returns -1. The expression botan_ffi_supports_api(botan_ffi_api_version()) will always evaluate to 0. A particular version of the library may also support other (older) versions of the FFI API.

const char *botan_version_string()

Returns a free-from version string, e.g., 2.0.0

uint32_t botan_version_major()

Returns the major version of the library

uint32_t botan_version_minor()

Returns the minor version of the library

uint32_t botan_version_patch()

Returns the patch version of the library

uint32_t botan_version_datestamp()

Returns the date this version was released as an integer, or 0 if an unreleased version

Utility Functions

int botan_same_mem(const uint8_t *x, const uint8_t *y, size_t len)

Returns 0 if x[0..len] == y[0..len], -1 otherwise.

int botan_hex_encode(const uint8_t *x, size_t len, char *out, uint32_t flags)

Performs hex encoding of binary data in x of size len bytes. The output buffer out must be of at least x*2 bytes in size. If flags contains BOTAN_FFI_HEX_LOWER_CASE, hex encoding will only contain lower-case letters, upper-case letters otherwise. Returns 0 on success, 1 otherwise.

Random Number Generators

typedef opaque *botan_rng_t

An opaque data type for a random number generator. Don’t mess with it.

int botan_rng_init(botan_rng_t *rng, const char *rng_type)

Initialize a random number generator object from the given rng_type: “system” or nullptr: System_RNG, “user”: AutoSeeded_RNG.

int botan_rng_get(botan_rng_t rng, uint8_t *out, size_t out_len)

Get random bytes from a random number generator.

int botan_rng_reseed(botan_rng_t rng, size_t bits)

Reseeds the random number generator with bits number of bits from the System_RNG.

int botan_rng_destroy(botan_rng_t rng)

Destroy the object created by botan_rng_init.

Block Ciphers

This is a ‘raw’ interface to ECB mode block ciphers. Most applications want the higher level cipher API which provides authenticated encryption. This API exists as an escape hatch for applications which need to implement custom primitives using a PRP.

typedef opaque *botan_block_cipher_t

An opaque data type for a block cipher. Don’t mess with it.

int botan_block_cipher_init(botan_block_cipher_t *bc, const char *cipher_name)

Create a new cipher mode object, cipher_name should be for example “AES-128” or “Threefish-512”

int botan_block_cipher_block_size(botan_block_cipher_t bc)

Return the block size of this cipher.

int botan_block_cipher_clear(botan_block_cipher_t bc)

Clear the internal state (such as keys) of this cipher object, but do not deallocate it.

int botan_block_cipher_set_key(botan_block_cipher_t bc, const uint8_t key[], size_t key_len)

Set the cipher key, which is required before encrypting or decrypting.

int botan_block_cipher_encrypt_blocks(botan_block_cipher_t bc, const uint8_t in[], uint8_t out[], size_t blocks)

The key must have been set first with botan_block_cipher_set_key. Encrypt blocks blocks of data stored in in and place the ciphertext into out. The two parameters may be the same buffer, but must not overlap.

int botan_block_cipher_decrypt_blocks(botan_block_cipher_t bc, const uint8_t in[], uint8_t out[], size_t blocks)

The key must have been set first with botan_block_cipher_set_key. Decrypt blocks blocks of data stored in in and place the ciphertext into out. The two parameters may be the same buffer, but must not overlap.

int botan_block_cipher_destroy(botan_block_cipher_t rng)

Destroy the object created by botan_block_cipher_init.

Hash Functions

typedef opaque *botan_hash_t

An opaque data type for a hash. Don’t mess with it.

botan_hash_t botan_hash_init(const char *hash, uint32_t flags)

Creates a hash of the given name, e.g., “SHA-384”. Returns null on failure. Flags should always be zero in this version of the API.

int botan_hash_destroy(botan_hash_t hash)

Destroy the object created by botan_hash_init.

int botan_hash_copy_state(botan_hash_t *dest, const botan_hash_t source)

Copies the state of the hash object to a new hash object.

int botan_hash_clear(botan_hash_t hash)

Reset the state of this object back to clean, as if no input has been supplied.

size_t botan_hash_output_length(botan_hash_t hash)

Return the output length of the hash function.

int botan_hash_update(botan_hash_t hash, const uint8_t *input, size_t len)

Add input to the hash computation.

int botan_hash_final(botan_hash_t hash, uint8_t out[])

Finalize the hash and place the output in out. Exactly botan_hash_output_length bytes will be written.

Message Authentication Codes

typedef opaque *botan_mac_t

An opaque data type for a MAC. Don’t mess with it, but do remember to set a random key first.

botan_mac_t botan_mac_init(const char *mac, uint32_t flags)

Creates a MAC of the given name, e.g., “HMAC(SHA-384)”. Returns null on failure. Flags should always be zero in this version of the API.

int botan_mac_destroy(botan_mac_t mac)

Destroy the object created by botan_mac_init.

int botan_mac_clear(botan_mac_t mac)

Reset the state of this object back to clean, as if no key and input have been supplied.

size_t botan_mac_output_length(botan_mac_t mac)

Return the output length of the MAC.

int botan_mac_set_key(botan_mac_t mac, const uint8_t *key, size_t key_len)

Set the random key.

int botan_mac_update(botan_mac_t mac, uint8_t buf[], size_t len)

Add input to the MAC computation.

int botan_mac_final(botan_mac_t mac, uint8_t out[], size_t *out_len)

Finalize the MAC and place the output in out. Exactly botan_mac_output_length bytes will be written.

Ciphers

typedef opaque *botan_cipher_t

An opaque data type for a MAC. Don’t mess with it, but do remember to set a random key first. And please use an AEAD.

botan_cipher_t botan_cipher_init(const char *cipher_name, uint32_t flags)

Create a cipher object from a name such as “AES-256/GCM” or “Serpent/OCB”.

Flags is a bitfield The low bit of flags specifies if encrypt or decrypt

int botan_cipher_destroy(botan_cipher_t cipher)
int botan_cipher_clear(botan_cipher_t hash)
int botan_cipher_set_key(botan_cipher_t cipher, const uint8_t *key, size_t key_len)
int botan_cipher_set_associated_data(botan_cipher_t cipher, const uint8_t *ad, size_t ad_len)
int botan_cipher_start(botan_cipher_t cipher, const uint8_t *nonce, size_t nonce_len)
int botan_cipher_is_authenticated(botan_cipher_t cipher)
size_t botan_cipher_tag_size(botan_cipher_t cipher)
int botan_cipher_valid_nonce_length(botan_cipher_t cipher, size_t nl)
size_t botan_cipher_default_nonce_length(botan_cipher_t cipher)

PBKDF

int botan_pbkdf(const char *pbkdf_algo, uint8_t out[], size_t out_len, const char *passphrase, const uint8_t salt[], size_t salt_len, size_t iterations)

Derive a key from a passphrase for a number of iterations using the given PBKDF algorithm, e.g., “PBKDF2”.

int botan_pbkdf_timed(const char *pbkdf_algo, uint8_t out[], size_t out_len, const char *passphrase, const uint8_t salt[], size_t salt_len, size_t milliseconds_to_run, size_t *out_iterations_used)

Derive a key from a passphrase using the given PBKDF algorithm, e.g., “PBKDF2”. If out_iterations_used is zero, instead the PBKDF is run until milliseconds_to_run milliseconds have passed. In this case, the number of iterations run will be written to out_iterations_used.

KDF

int botan_kdf(const char *kdf_algo, uint8_t out[], size_t out_len, const uint8_t secret[], size_t secret_len, const uint8_t salt[], size_t salt_len, const uint8_t label[], size_t label_len)

Derive a key using the given KDF algorithm, e.g., “SP800-56C”. The derived key of length out_len bytes will be placed in out.

Multiple Precision Integers

typedef opaque *botan_mp_t

An opaque data type for a multiple precision integer. Don’t mess with it.

int botan_mp_init(botan_mp_t *mp)

Initialize a botan_mp_t. Initial value is zero, use botan_mp_set_X to load a value.

int botan_mp_destroy(botan_mp_t mp)

Free a botan_mp_t

int botan_mp_to_hex(botan_mp_t mp, char *out)

Writes exactly botan_mp_num_bytes(mp)*2 + 1 bytes to out

int botan_mp_to_str(botan_mp_t mp, uint8_t base, char *out, size_t *out_len)

Base can be either 10 or 16.

int botan_mp_set_from_int(botan_mp_t mp, int initial_value)

Set botan_mp_t from an integer value.

int botan_mp_set_from_mp(botan_mp_t dest, botan_mp_t source)

Set botan_mp_t from another MP.

int botan_mp_set_from_str(botan_mp_t dest, const char *str)

Set botan_mp_t from a string. Leading prefix of “0x” is accepted.

int botan_mp_num_bits(botan_mp_t n, size_t *bits)

Return the size of n in bits.

int botan_mp_num_bytes(botan_mp_t n, size_t *uint8_ts)

Return the size of n in bytes.

int botan_mp_to_bin(botan_mp_t mp, uint8_t vec[])

Writes exactly botan_mp_num_bytes(mp) to vec.

int botan_mp_from_bin(botan_mp_t mp, const uint8_t vec[], size_t vec_len)

Loads botan_mp_t from a binary vector (as produced by botan_mp_to_bin).

int botan_mp_is_negative(botan_mp_t mp)

Return 1 if mp is negative, otherwise 0.

int botan_mp_flip_sign(botan_mp_t mp)

Flip the sign of mp.

int botan_mp_add(botan_mp_t result, botan_mp_t x, botan_mp_t y)

Add two botan_mp_t``s and store the output in ``result.

int botan_mp_sub(botan_mp_t result, botan_mp_t x, botan_mp_t y)

Subtract two botan_mp_t``s and store the output in ``result.

int botan_mp_mul(botan_mp_t result, botan_mp_t x, botan_mp_t y)

Multiply two botan_mp_t``s and store the output in ``result.

int botan_mp_div(botan_mp_t quotient, botan_mp_t remainder, botan_mp_t x, botan_mp_t y)

Divide x by y and store the output in quotient and remainder.

int botan_mp_mod_mul(botan_mp_t result, botan_mp_t x, botan_mp_t y, botan_mp_t mod)

Set result to x times y modulo mod.

int botan_mp_equal(botan_mp_t x, botan_mp_t y)

Return 1 if x is equal to y, 0 if x is not equal to y

int botan_mp_is_zero(const botan_mp_t x)

Return 1 if x is equal to zero, otherwise 0.

int botan_mp_is_odd(const botan_mp_t x)

Return 1 if x is odd, otherwise 0.

int botan_mp_is_even(const botan_mp_t x)

Return 1 if x is even, otherwise 0.

int botan_mp_is_positive(const botan_mp_t x)

Return 1 if x is greater than or equal to zero.

int botan_mp_is_negative(const botan_mp_t x)

Return 1 if x is less than zero.

int botan_mp_to_uint32(const botan_mp_t x, uint32_t *val)

If x fits in a 32-bit integer, set val to it and return 0. If x is out of range an error is returned.

int botan_mp_cmp(int *result, botan_mp_t x, botan_mp_t y)

Three way comparison: set result to -1 if x is less than y, 0 if x is equal to y, and 1 if x is greater than y.

int botan_mp_swap(botan_mp_t x, botan_mp_t y)

Swap two botan_mp_t values.

int botan_mp_powmod(botan_mp_t out, botan_mp_t base, botan_mp_t exponent, botan_mp_t modulus)

Modular exponentiation.

int botan_mp_lshift(botan_mp_t out, botan_mp_t in, size_t shift)

Left shift by specified bit count, place result in out.

int botan_mp_rshift(botan_mp_t out, botan_mp_t in, size_t shift)

Right shift by specified bit count, place result in out.

int botan_mp_mod_inverse(botan_mp_t out, botan_mp_t in, botan_mp_t modulus)

Compute modular inverse. If no modular inverse exists (for instance because in and modulus are not relatively prime), then sets out to -1.

int botan_mp_rand_bits(botan_mp_t rand_out, botan_rng_t rng, size_t bits)

Create a random botan_mp_t of the specified bit size.

int botan_mp_rand_range(botan_mp_t rand_out, botan_rng_t rng, botan_mp_t lower_bound, botan_mp_t upper_bound)

Create a random botan_mp_t within the provided range.

int botan_mp_gcd(botan_mp_t out, botan_mp_t x, botan_mp_t y)

Compute the greatest common divisor of x and y.

int botan_mp_is_prime(botan_mp_t n, botan_rng_t rng, size_t test_prob)

Test if n is prime. The algorithm used (Miller-Rabin) is probabilistic, set test_prob to the desired assurance level. For example if test_prob is 64, then sufficient Miller-Rabin iterations will run to assure there is at most a 1/2**64 chance that n is composite.

int botan_mp_get_bit(botan_mp_t n, size_t bit)

Returns 0 if the specified bit of n is not set, 1 if it is set.

int botan_mp_set_bit(botan_mp_t n, size_t bit)

Set the specified bit of n

int botan_mp_clear_bit(botan_mp_t n, size_t bit)

Clears the specified bit of n

Password Hashing

int botan_bcrypt_generate(uint8_t *out, size_t *out_len, const char *password, botan_rng_t rng, size_t work_factor, uint32_t flags)

Create a password hash using Bcrypt. The output buffer out should be of length 64 bytes. The output is formatted bcrypt $2a$…

int botan_bcrypt_is_valid(const char *pass, const char *hash)

Check a previously created password hash. Returns 0 if if this password/hash combination is valid, 1 if the combination is not valid (but otherwise well formed), negative on error.

Public Key Creation, Import and Export

typedef opaque *botan_privkey_t

An opaque data type for a private key. Don’t mess with it.

int botan_privkey_create(botan_privkey_t *key, const char *algo_name, const char *algo_params, botan_rng_t rng)
int botan_privkey_create_rsa(botan_privkey_t *key, botan_rng_t rng, size_t n_bits)
int botan_privkey_create_ecdsa(botan_privkey_t *key, botan_rng_t rng, const char *params)
int botan_privkey_create_ecdh(botan_privkey_t *key, botan_rng_t rng, const char *params)
int botan_privkey_create_mceliece(botan_privkey_t *key, botan_rng_t rng, size_t n, size_t t)
int botan_privkey_create_dh(botan_privkey_t *key, botan_rng_t rng, const char *params)
int botan_privkey_load(botan_privkey_t *key, botan_rng_t rng, const uint8_t bits[], size_t len, const char *password)
int botan_privkey_destroy(botan_privkey_t key)
int botan_privkey_export(botan_privkey_t key, uint8_t out[], size_t *out_len, uint32_t flags)
int botan_privkey_export_encrypted(botan_privkey_t key, uint8_t out[], size_t *out_len, botan_rng_t rng, const char *passphrase, const char *encryption_algo, uint32_t flags)

Deprecated, use botan_privkey_export_encrypted_msec or botan_privkey_export_encrypted_iter

typedef opaque *botan_pubkey_t

An opaque data type for a public key. Don’t mess with it.

int botan_pubkey_load(botan_pubkey_t *key, const uint8_t bits[], size_t len)
int botan_privkey_export_pubkey(botan_pubkey_t *out, botan_privkey_t in)
int botan_pubkey_export(botan_pubkey_t key, uint8_t out[], size_t *out_len, uint32_t flags)
int botan_pubkey_algo_name(botan_pubkey_t key, char out[], size_t *out_len)
int botan_pubkey_estimated_strength(botan_pubkey_t key, size_t *estimate)
int botan_pubkey_fingerprint(botan_pubkey_t key, const char *hash, uint8_t out[], size_t *out_len)
int botan_pubkey_destroy(botan_pubkey_t key)
int botan_pubkey_get_field(botan_mp_t output, botan_pubkey_t key, const char *field_name)

Read an algorithm specific field from the public key object, placing it into output. For example “n” or “e” for RSA keys or “p”, “q”, “g”, and “y” for DSA keys.

int botan_privkey_get_field(botan_mp_t output, botan_privkey_t key, const char *field_name)

Read an algorithm specific field from the private key object, placing it into output. For example “p” or “q” for RSA keys, or “x” for DSA keys or ECC keys.

RSA specific functions

int botan_privkey_rsa_get_p(botan_mp_t p, botan_privkey_t rsa_key)

Set p to the first RSA prime.

int botan_privkey_rsa_get_q(botan_mp_t q, botan_privkey_t rsa_key)

Set q to the second RSA prime.

int botan_privkey_rsa_get_d(botan_mp_t d, botan_privkey_t rsa_key)

Set d to the RSA private exponent.

int botan_privkey_rsa_get_n(botan_mp_t n, botan_privkey_t rsa_key)

Set n to the RSA modulus.

int botan_privkey_rsa_get_e(botan_mp_t e, botan_privkey_t rsa_key)

Set e to the RSA public exponent.

int botan_pubkey_rsa_get_e(botan_mp_t e, botan_pubkey_t rsa_key)

Set e to the RSA public exponent.

int botan_pubkey_rsa_get_n(botan_mp_t n, botan_pubkey_t rsa_key)

Set n to the RSA modulus.

int botan_privkey_load_rsa(botan_privkey_t *key, botan_mp_t p, botan_mp_t q, botan_mp_t e)

Initialize a private RSA key using parameters p, q, and e.

int botan_pubkey_load_rsa(botan_pubkey_t *key, botan_mp_t n, botan_mp_t e)

Initialize a public RSA key using parameters n and e.

DSA specific functions

int botan_privkey_load_dsa(botan_privkey_t *key, botan_mp_t p, botan_mp_t q, botan_mp_t g, botan_mp_t x)

Initialize a private DSA key using group parameters p, q, and g and private key x.

int botan_pubkey_load_dsa(botan_pubkey_t *key, botan_mp_t p, botan_mp_t q, botan_mp_t g, botan_mp_t y)

Initialize a private DSA key using group parameters p, q, and g and public key y.

ElGamal specific functions

int botan_privkey_load_elgamal(botan_privkey_t *key, botan_mp_t p, botan_mp_t g, botan_mp_t x)

Initialize a private ElGamal key using group parameters p and g and private key x.

int botan_pubkey_load_elgamal(botan_pubkey_t *key, botan_mp_t p, botan_mp_t g, botan_mp_t y)

Initialize a public ElGamal key using group parameters p and g and public key y.

Diffie-Hellman specific functions

int botan_privkey_load_dh(botan_privkey_t *key, botan_mp_t p, botan_mp_t g, botan_mp_t x)

Initialize a private Diffie-Hellman key using group parameters p and g and private key x.

int botan_pubkey_load_dh(botan_pubkey_t *key, botan_mp_t p, botan_mp_t g, botan_mp_t y)

Initialize a public Diffie-Hellman key using group parameters p and g and public key y.

Public Key Encryption/Decryption

typedef opaque *botan_pk_op_encrypt_t

An opaque data type for an encryption operation. Don’t mess with it.

int botan_pk_op_encrypt_create(botan_pk_op_encrypt_t *op, botan_pubkey_t key, const char *padding, uint32_t flags)
int botan_pk_op_encrypt_destroy(botan_pk_op_encrypt_t op)
int botan_pk_op_encrypt(botan_pk_op_encrypt_t op, botan_rng_t rng, uint8_t out[], size_t *out_len, const uint8_t plaintext[], size_t plaintext_len)
typedef opaque *botan_pk_op_decrypt_t

An opaque data type for a decryption operation. Don’t mess with it.

int botan_pk_op_decrypt_create(botan_pk_op_decrypt_t *op, botan_privkey_t key, const char *padding, uint32_t flags)
int botan_pk_op_decrypt_destroy(botan_pk_op_decrypt_t op)
int botan_pk_op_decrypt(botan_pk_op_decrypt_t op, uint8_t out[], size_t *out_len, uint8_t ciphertext[], size_t ciphertext_len)

Signatures

typedef opaque *botan_pk_op_sign_t

An opaque data type for a signature generation operation. Don’t mess with it.

int botan_pk_op_sign_create(botan_pk_op_sign_t *op, botan_privkey_t key, const char *hash_and_padding, uint32_t flags)
int botan_pk_op_sign_destroy(botan_pk_op_sign_t op)
int botan_pk_op_sign_update(botan_pk_op_sign_t op, const uint8_t in[], size_t in_len)
int botan_pk_op_sign_finish(botan_pk_op_sign_t op, botan_rng_t rng, uint8_t sig[], size_t *sig_len)
typedef opaque *botan_pk_op_verify_t

An opaque data type for a signature verification operation. Don’t mess with it.

int botan_pk_op_verify_create(botan_pk_op_verify_t *op, botan_pubkey_t key, const char *hash_and_padding, uint32_t flags)
int botan_pk_op_verify_destroy(botan_pk_op_verify_t op)
int botan_pk_op_verify_update(botan_pk_op_verify_t op, const uint8_t in[], size_t in_len)
int botan_pk_op_verify_finish(botan_pk_op_verify_t op, const uint8_t sig[], size_t sig_len)

Key Agreement

typedef opaque *botan_pk_op_ka_t

An opaque data type for a key agreement operation. Don’t mess with it.

int botan_pk_op_key_agreement_create(botan_pk_op_ka_t *op, botan_privkey_t key, const char *kdf, uint32_t flags)
int botan_pk_op_key_agreement_destroy(botan_pk_op_ka_t op)
int botan_pk_op_key_agreement_export_public(botan_privkey_t key, uint8_t out[], size_t *out_len)
int botan_pk_op_key_agreement(botan_pk_op_ka_t op, uint8_t out[], size_t *out_len, const uint8_t other_key[], size_t other_key_len, const uint8_t salt[], size_t salt_len)
int botan_mceies_encrypt(botan_pubkey_t mce_key, botan_rng_t rng, const char *aead, const uint8_t pt[], size_t pt_len, const uint8_t ad[], size_t ad_len, uint8_t ct[], size_t *ct_len)
int botan_mceies_decrypt(botan_privkey_t mce_key, const char *aead, const uint8_t ct[], size_t ct_len, const uint8_t ad[], size_t ad_len, uint8_t pt[], size_t *pt_len)

X.509 Certificates

typedef opaque *botan_x509_cert_t

An opaque data type for an X.509 certificate. Don’t mess with it.

int botan_x509_cert_load(botan_x509_cert_t *cert_obj, const uint8_t cert[], size_t cert_len)
int botan_x509_cert_load_file(botan_x509_cert_t *cert_obj, const char *filename)
int botan_x509_cert_destroy(botan_x509_cert_t cert)
int botan_x509_cert_gen_selfsigned(botan_x509_cert_t *cert, botan_privkey_t key, botan_rng_t rng, const char *common_name, const char *org_name)
int botan_x509_cert_get_time_starts(botan_x509_cert_t cert, char out[], size_t *out_len)
int botan_x509_cert_get_time_expires(botan_x509_cert_t cert, char out[], size_t *out_len)
int botan_x509_cert_get_fingerprint(botan_x509_cert_t cert, const char *hash, uint8_t out[], size_t *out_len)
int botan_x509_cert_get_serial_number(botan_x509_cert_t cert, uint8_t out[], size_t *out_len)
int botan_x509_cert_get_authority_key_id(botan_x509_cert_t cert, uint8_t out[], size_t *out_len)
int botan_x509_cert_get_subject_key_id(botan_x509_cert_t cert, uint8_t out[], size_t *out_len)
int botan_x509_cert_path_verify(botan_x509_cert_t cert, const char *ca_dir)
int botan_x509_cert_get_public_key_bits(botan_x509_cert_t cert, uint8_t out[], size_t *out_len)
int botan_x509_cert_get_public_key(botan_x509_cert_t cert, botan_pubkey_t *key)
int botan_x509_cert_get_issuer_dn(botan_x509_cert_t cert, const char *key, size_t index, uint8_t out[], size_t *out_len)
int botan_x509_cert_get_subject_dn(botan_x509_cert_t cert, const char *key, size_t index, uint8_t out[], size_t *out_len)
int botan_x509_cert_to_string(botan_x509_cert_t cert, char out[], size_t *out_len)
enum botan_x509_cert_key_constraints

Certificate key usage constraints. Allowed values: NO_CONSTRAINTS, DIGITAL_SIGNATURE, NON_REPUDIATION, KEY_ENCIPHERMENT, DATA_ENCIPHERMENT, KEY_AGREEMENT, KEY_CERT_SIGN, CRL_SIGN, ENCIPHER_ONLY, DECIPHER_ONLY.

int botan_x509_cert_allowed_usage(botan_x509_cert_t cert, unsigned int key_usage)